Some installs break due to unzip lowercasing filenames. Detect this and print a nice error

See this post: http://moodle.org/mod/forum/discuss.php?d=65786#p670587

Moodle just failing to install with a white-screen of death gives a bad impression to new users. Since this problem seems to occur relatively frequently, it might be worth detecting it specifically, and printing a nice message to explain the problem to users.